Piles are used to support structures, such as buildings, when the soil underlying the structure is too weak to support the structure. There are many techniques that may be used to place a pile. One technique is to cast the pile in place. In this technique, a hole is excavated in the place where the pile is needed and the hole is filled with cement. A problem with this technique is that in weak soils the hole tends to collapse. Therefore, expensive shoring is required. If the hole is more than about 4 to 5 feet deep then safety regulations typically require expensive shoring and other safety precautions to prevent workers from being trapped in the hole.
Turzillo, U.S. Pat. No. 3,962,879 is a modification of this technique. In the Turzillo system a helical auger is used to drill a cylindrical cavity in the earth. The upper end of the auger is held fixed while the auger is rotated about its axis to remove all of the earth from the cylindrical cavity. After the earth has been removed fluid cement water is pumped through the shaft of the auger until the hole is filled with cement. The auger is left in place. Turzillo, U.S. Pat. No. 3,354,657 shows a similar system.
Langenbach Jr., U.S. Pat. No. 4,678,373 discloses a method for supporting a structure in which a piling bearing a footing structure is driven down into the ground by pressing from above with a large hydraulic ram anchored to the structure. The void cleared by the footing structure may optionally be filled by pumping concrete into the void through a channel inside the pile. The ram used to insert the Langenbach Jr. piling is large, heavy and expensive.
Another approach to placing piles is to insert a hollow form in the ground with the piles desired and then to fill the hollow form with fluid cement. Hollow forms may be driven into the ground by impact or screwed into the ground. This approach is cumbersome because the hollow forms are unwieldy and expensive. Examples of this approach are described in U.S. Pat. Nos. 2,326,872 and 2,926,500.
Helical pier systems, such as the CHANCE.RTM. helical pier system available from the A.B. Chance Company of Centralia Mo. U.S.A., provide an attractive alternative to the systems described above. As described in more detail below, the CHANCE helical pier system includes one or more helical screws mounted at the end of a shaft. The helical screw comprises a section of metal plate having its inner edge welded to the shaft. The area around the inner edge is the root region of the screw. The plate is bent so that its outer edge generally follows a helix. The shaft is turned to draw the helical screw downwardly into a body of soil. The screw is screwed downwardly until the screw is seated in a region of soil sufficiently strong to support the weight which will be placed on the pier.
Brackets may be mounted on the upper end of the pier to support the foundation of a building. Helical pier systems have the advantages that they are relatively inexpensive to use and are relatively easy to install in tight quarters. Helical pier systems have two primary disadvantages. Firstly, they rely upon the surrounding soil to support the shaft and to prevent the shaft from bending. In situation where the surrounding soil is very weak or the pier is required to support very large loads the surrounding soil cannot provide the necessary support. Consequently, helical piers can bend in such situations. A second disadvantage of helical piers is that the metal components of the piers are in direct contact with the surrounding soil. Consequently, if the shaft passes through regions in the soil which are highly chemically active then the shaft may be eroded, thereby weakening the pier. A third disadvantage of helical piers exists in piers which comprise large diameter helices which bear large loads. Such helices can buckle and cause the pier to fail. Because their load bearing capacity is limited, helical pier systems have not been able to replace more conventional piles in many applications.
There is a need for a relatively inexpensive method for forming piles without the use of heavy expensive equipment which overcomes at least some of the above-noted disadvantages of helical piers.